This invention relates generally to golf clubs and, more particularly, to a hybrid shaft for improving the performance of golf clubs.
A modern golf club typically comprises a head connected to a shaft, and a gripping region disposed on the end of the shaft opposite the head. Perhaps more than any other component, the shaft affects overall club performance. It is generally accepted that the optimum golf club shaft should have the following characteristics: (1) lightweight for high swing velocity; (2) high torsional stiffness to limit unwanted angular deflection of the head about the shaft; (3) configurable bending stiffness; (4) moderate high swing weights; and (5) energy-absorbing ability to soften shocks from miss-hits and ground strikes. These characteristics are described below in greater detail.
1. Lightweight
All golfers benefit from a lightweight club. A lightweight club will have greater acceleration for the same applied force than a heavier club. Greater acceleration equates to a higher swing velocity. Swing velocity is an important factor in driving a ball: for clubs of similar weight and mass distribution, the greater the swing velocity, the farther the ball will travel. Therefore, lighter clubs are preferable from the perspective of swing velocity.
2. High Torsional Stiffness
All golfers benefit from a torsionally-stiff shaft. The center of mass of a club head is offset from the axis of the shaft. Thus, when the club head is accelerated during the swing, inertial forces will tend to rotate the club head about the shaft axis, twisting the shaft elastically in inverse proportion to the shaft""s torsional stiffness. As a result, the face of the club head does not meet the ball squarely; rather, the club head xe2x80x9ctoesxe2x80x9d outward thereby meeting the ball at an angle. This causes the ball""s flight to veer from a straight path. It is thus desirable to have the shaft as torsionally stiff as practicable to limit the adverse effects of club head rotation
The torsional stiffness of a hollow, closed section such as used for golf club shafts is proportional to both the polar moment of inertia of the section and the shear modulus of the material forming the shaft. For example, larger diameter shafts have larger polar moments of inertia and are significantly stiffer in torsion than smaller diameter sections formed from the same material. Likewise shafts formed from a material such as steel, which has a relatively high shear modulus, are inherently stiffer in torsion than a shaft with the same dimension formed from graphite which has a lower shear modulus.
3. Moderate Swing Weight
Swing weight is a measure of how the mass is distributed on a club and equates to the dynamic characteristics or xe2x80x9cfeelxe2x80x9d of the club. Different clubs having different lengths and weights but having the same or similar swing weight will feel the same to the golfer when swung. To achieve consistent play it is important that the various clubs feel the same or at least closely similar during the swing. The swing weight parameter allows a golfer to assemble a set of clubs best suited to his particular needs by matching the dynamic characteristics (the feel) of the various clubs in the set for consistency of feel and play by matching club swing weights.
Weight concentrated toward the head of the club will tend to increase swing weight while weight concentrated toward the butt end of a club tends to decrease swing weight. Swing weight is measured on a scale for A-F, with A being the lightest swing weight and F being the heaviest. Although some golfers prefer heavy swing weights, most prefer moderate swing weights in the range of D-E.
4. Configurable Bending Stiffness
It is important to match the bending stiffness of the club to the abilities of the player. Professional golfers who are able to generate relatively high swing velocity for maximum driving distance tend to prefer clubs having a relatively high bending stiffness. On the other hand, those golfers who generate lower swing velocity tend to prefer a club with relatively low bending stiffness to take advantage of the xe2x80x9ckickxe2x80x9d resulting from the flexing of the shaft during the early part of the swing and the subsequent release as the golf club head squares with the ball. Thus, it is desirable to have a golf club design which affords a wide range of bending stiffness to accommodate the different needs of various players.
5. Vibration Damping
A club should absorb shock and vibration caused by the head striking the ball and/or ground. Absent such dampening, the shock is transmitted up the shaft and to the user""s hands. This can be problematic, especially for those troubled with arthritis.
Conventional club shaft designs have addressed a few of the club characteristics noted above, although no one shaft design has satisfactorily addressed all of these important characteristics. The applicants are aware of essentially three conventional shaft designs: (1) a steel shaft; (2) a graphite shaft; and (3) a hybrid shaft of graphite and steel. Although these designs offer certain advantages, they tend to optimize some of the characteristics mentioned above while compromising others as described below.
1. Steel Shaft
The steel shaft has long been the mainstay of golf club design. The steel shaft provides several advantages. Steel has a high shear modulus which results in shafts having an inherently high torsional stiffness which greatly limits undesired club head rotation or toe out. A wide range of bending stiffness and swing weights can be obtained with the steel shaft by controlling the relative lengths of the smaller diameter sections of the shaft near the club head, with a more flexible shaft being provided by increasing the lengths of the more flexible, smaller diameter sections while reducing the lengths of the relatively stiffer, larger diameter sections. Steel is also durable, strong, inexpensive to manufacture, and provides great consistency of characteristics from one shaft to another.
Unfortunately, steel is dense, and clubs having steel shafts are heavy, have relatively poor acceleration and consequently a lower swing velocity. Additionally, The conventional rubber grip used with the steel shaft also contributes to the weight problem. It is a relatively heavy part of the club, representing, for example, about 15% of the total mass of a typical driver or any fairway wood. These effects are amplified for an oversized grip which are used commonly by people with arthritis or large hands.
Aside from being heavy, steel shafts also tend not to absorb or dampen vibration. Consequently, shocks tend to be transferred from the club head to the user""s hands along the shaft.
Thus, although the steel shaft has some advantages, the main advantages being its wide range of bending stiffness and its high torsional stiffness, it also has serious disadvantages of being heavy and poor at absorbing or dampening shock and vibration.
2. Graphite Shaft
Clubs with composite shafts such as graphite are an improvement over steel-shafted clubs in two respects: (1) graphite is substantially less dense than steel yielding a significantly lighter shaft; and (2) a graphite shaft can absorb shock and vibration much better than a steel shaft. A lighter shaft reduces the overall weight of the club and results in higher swing velocity, which produces longer drives as explained above.
The lightweight nature of the graphite shafts are enhanced by the elimination of the rubber grip. A gripless graphite shaft does not have a separate element forming the grip, but rather, the grip is an integral part of the shaft formed by wrapping the graphite over a conically shaped mandrel having a relatively large diameter over a predetermined length at the butt end of the club. The butt end of the shaft thus has a tapered cross section and acts like the conical wedge of the conventional rubber grip to provide a comfortable and secure grip to the golfer. The shaft butt is wrapped over the length of the enlarged diameter with a thin plastic tape to form a frictional gripping surface.
The primary drawbacks of the composite graphite design are its high bending stiffness and low torsional stiffness which is a result of how the shaft is fabricated. In order to achieve the necessary bending strength and light weight of the shaft, unidirectional graphite fibers bound in a resin matrix are helically wrapped or wound around a mandrel in layers which are then cured under heat and pressure to form the shaft. The fibers are wrapped at a relatively high helix angle which orients the fibers as closely as practicable along the length of the shaft to take advantage of the high tensile strength of the graphite fibers and provide strength in bending. However, such large helical wrap angles result in low torsional rigidity largely because the fibers are not oriented circumferentially and therefore cannot effectively resist torsional deflections of the shaft.
The characteristic inaccuracy associated with graphite shafts can be mitigated by angling the face of the golf club""s head in a direction opposite of the shaft""s twist. For example, the club face would have a counterclockwise angle for a right-handed club. This angle compensates for the shaft""s torsional twist such that, upon impact, the club""s momentum transfers substantially squarely to the ball. Such compensation, however, is imprecise. The amount of compensation varies not only according to the user, but also according to the strength of a user""s particular swing. Consequently, serious golfers prefer not to rely on such compensation. In general, professional golfers do not use graphite shaft clubs but rather continue to use clubs with steel shafts.
Although the graphite shaft provides advantages such as the ability to absorb the shock and vibration of miss-hit balls or ground strikes and a lighter weight club resulting in higher swing velocity, the low torsional stiffness and high bending stiffness of the club presents serious disadvantages which most professional golfers find unacceptable.
3. Hybrid Shaft
Although not commercialized, a hybrid shaft disclosed in Pompa, U.S. Pat. No. 4,836,545, combines the advantages of lightweight and good vibration damping associated with a graphite shaft with the advantages bending flexibility and torsional stiffness of a steel shaft by joining together a graphite butt end shaft section with a steel head end shaft section.
Unfortunately, it has been found that a club of this design has an unacceptably-high swing weight. More specifically, the weight of the hybrid shaft club is concentrated at the head end since the shaft near the club comprises a heavy conventional steel section while the shaft near the butt end comprises a lightweight graphite section. As mentioned above, a high swing weight gives a club a xe2x80x9cheavy,xe2x80x9d undesirable feel in the user""s hands. Thus, for the hybrid shaft, the advantage of reduced overall club weight, good shock and vibration absorption, and high torsional has been achieved at the expense of an increased and undesired swing weight.
An overall comparative summary of conventional shaft designs is provided in Table 1 below.
Thus, there is a need for a shaft that possesses the attributes indicated above without compromising others. The present invention fulfills this need among others.
The present invention provides for a golf club having a shaft of multiple sections which have a linear weight less than that of a conventional steel shaft and which are configured to contribute different properties to the club such that optimal overall club performance is achieved. In particular, high torsional stiffness and moderate swing weight are achieved synergistically by configuring the narrow section of the shaft that connects to the hossel of the head such that its linear weight is less than that of a conventional steel shaft while maintaining comparable torsional stiffness. It has been found that linear weight may be decreased while maintaining torsional stiffness by exploiting the difference between linear weight and torsional stiffness as functions of wall thickness and diameter. That is, for a given wall thickness, torsional stiffness increases more than linear weight for a given increase in diameter. Furthermore, torsional stiffness can be increased by constructing the section of a relatively-high shear modulus material such as steel. Thus, a relatively-low linear weight section with torsional stiffness comparable to that of a conventional steel shaft can be provided by increasing shaft diameter and reducing wall thickness in the proper proportions.
To reduce club weight, a majority section of the shaft comprises a lightweight material such as graphite. This section also may have a conically-shaped butt end with an enlarged diameter to provide a comfortable and secure grip for the user without the need for a conventional grip which adds considerable weight to the club. The lightweight shaft translates to greater swing velocity and commensurately further distance on a drive.
Improved vibration dampening is achieved through the use of known energy-absorbing materials in the shaft sections. A synergistic result is realized if the lightweight material used in the majority section of the shaft is also energy absorbing as is graphite. Furthermore, the use of a connector for joining the shaft to the hossel of the head has been found to be effective in dampening vibration, particularly if it is formed of an energy absorbing material like graphite. This connector also has the synergistic feature of dispersing load along a greater area of the shaft section, thereby reducing stress at the joint of the shaft and head. Vibration dampening also may be improved through the use of one or more stiffeners or plugs which are disposed in a shaft section to resist radial deformation thereto.
Variable bending stiffness is achieved by varying the relative lengths of the sections. More specifically, since the section near the hossel of the club is the most narrow part of the shaft and preferably comprises a bendable material such as steel, the relative length and diameter of this section determines the overall flexibility of the shaft. Accordingly, if a more flexible or stiffer club is desired, then the length of this section can be increased or decreased respectively. Furthermore, it has been found that the bending performance of the shaft can be adjusted through the use of one or more stiffeners as mentioned above. Thus, stiffeners have the synergistic result of not only dampening vibration but also stiffening the club, particularly if disposed in the narrow section of the shaft.
Thus, in accordance with the present invention, by controlling the relative lengths, wall thicknesses and material properties of the shaft sections, a club can be configured having the lightweight and vibration damping of a graphite shaft, as well as the wide range of bending stiffness properties and high torsional stiffness of a steel shaft without an excessively high swing weight. With respect to the comparison in Table 1, the club of the present invention has a very light total weight, a moderate D5 to E5 swing weight, excellent (variable) bending stiffness, excellent torsional stiffness, and excellent vibration dampening.
One aspect of the invention is a shaft for attachment to a club comprising sections of different material with a low-weight section connected to the head. In a preferred embodiment, the said shaft comprises: (a) a first section comprising a first material and having a hossel end and a first joint end, the first section having a linear weight no greater than 2.4 g/in; (b) a second section comprising a second material and having a butt end and a second joint end, the second joint end being connected to the first joint end; (c) wherein the second material is less dense than the first material; and (d) wherein the first material has a shear modulus greater than that of the second material.
Another aspect of the invention is a golf club having the shaft as described above. In a preferred embodiment, the golf club comprises: (a) a head having a hossel; (b) a first section comprising a first material and having a hossel end and a first joint end, the first section having a linear weight no greater than 2.4 g/in; (c) a connector for connecting the first section to the hossel; (d) a second section comprising a second material and having a butt end and a second joint end, the second joint end being connected to the first joint end; (e) wherein the second material is less dense than the first material; and (f) wherein the first material has a shear modulus greater than that of the second material.
Yet another aspect is a method of modifying a conventional graphite shaft with a custom section near the hossel. In a preferred embodiment, the method comprises: (a) providing a first section of shaft having a linear weight no greater than about 2.4 g/in, and comprising a first material having a shear modulus greater than that of graphite; (b) providing a graphite shaft having a butt end and a hossel end; (c) removing a certain length of the graphite shaft from its hossel end; and (d) interengaging the first section with the end of the graphite shaft from which the certain length of shaft was removed.
Still another aspect of the present invention is a customized section adapted for connection to a hossel of a club head and a section of a graphite shaft. In a preferred embodiment, the customized section has a linear weight no greater than about 2.4 g/in and comprises a first material having a shear modulus greater than that of graphite.